This invention relates to improved electroluminescent lamps and phosphors, and to a method of making such improved phosphors.
Conventional electroluminescent lamps have panels which are made with electroluminescent phosphors, such as copper activated zinc sulfide, embedded in a resin layer between a pair of electrodes. Such conventional lamps suffer from aging and degradation due to moisture, such as by the migration of water molecules into the matrix of the phosphor crystals. The aging process is accompanied by a loss in brightness, for a given level of excitation, and a shift in color of the lamp, both in the lamp's lighted and unlighted status. As a result, it has become necessary to go to extraordinary lengths to protect such lamp and panels against moisture.
The lighted panels of conventional lamps also do not lend themselves to manufacturing processes which include die cutting, punching, perforating, or trimming through the active phosphor layer, as such operations will either immediately short out the lamp, or will result in a premature loss of brightness in the vicinity of the cut, and often accompanied by eventual failure of the entire lamp. Such behavior of conventional lamps severely restrict the use of electroluminescent lamps in many commercial applications which require punching, die cutting or the like, in a low-cost and mass-produced panel and/or in which moisture is present.
Electroluminescent lamps which lack extraordinary external protection against the infusion of moisture are not only prone to suffer from loss of output, i.e., aging, but as noted above are observed to have a shift toward the color pink where the lamp output was originally white. Further, where the natural color of such a lamp in its unlighted condition is an overall light tan, such lamps have been observed, with aging, to take on an overall gray or black color. In many cases, such a change in color is undesirable or unacceptable.
The desirability of encapsulating electroluminescent phosphors to retard aging has been recognized. Both organic and inorganic coatings have been suggested, with varying degrees of success. One approach, as disclosed in the patents of Allinikov, U.S. Pat. No. 4,097,776 issued June 27, 1978 and Olson et al, U.S. Pat. No. 4,508,760 issued Apr. 2, 1985, includes the use of organic or polymer resin materials for encapsulation. In Allinikov, the phosphor particles are immersed in a solution of liquid crystal material and stirred, and thereafter dried to form a resin coating. In Olson et al, specific polymers are vacuum deposited on the surface of the crystals.
Resin coated phosphors have not found general use in the manufacture of electroluminescent lamps since they suffer from many of the same problems as do conventional resin embedded particles, that is, that the resins do not fully exclude moisture and may interact with the phosphor. When such resin encapsulated particles are used as a substitute for conventional uncoated particles, they may be mixed with a resin adhesive and applied, as by screen printing or by a blade, to a substrate in the manufacture of the lamp. The intermediate resin coating surrounding the particles is usually no better in preventing aging than is the adhesive or casting resin itself.
The prior art also contains a number of attempts to provide an inorganic barrier or coating on the phosphor particles, including Piper, U.S. Pat. No. 2,944,177 issued July 5, 1960. In Piper, phosphor crystals or particles are mixed with a glass frit, and then heated to approximately 530.degree. F. until the glass fuses, producing a phosphor and glass agglomerate. This is then cooled and crushed until the resulting particles are sufficiently small so as to be applied as a glass coated particle in lieu of conventional electroluminescent phosphor grains. However, the glass fusing and crushing process of Piper has not come into general usage because of two principal disadvantages. First, in crushing or grinding, many of the phosphor particles themselves are ruptured or broken and exposed, and are therefore subject to the normal effects of aging. Further, the process produces too much glass in relation to the phosphor content, without close control of the thickness of the glass deposition with respect to the phosphor particles.
Brooks, U.S. Pat. No. 3,264,133 of Aug. 2, 1966 discloses the coating of the phosphor particles with an inorganic coatings, such as barium titanate and titanium dioxide, to provide a high dielectric coating. While Brooks achieves an enhancement in brightness due to an increase in dielectric constant, it is not apparent that these coatings are useful to extend the life of the phosphor or exclude moisture.
In a number of related patents, Fischer has described the aging process in zinc sulfide phosphors and provides recipes for rendering the phosphors less immune to aging and for coating the phosphors with inorganic phosphates. These patents include U.S. Pat. Nos. 4,143,297 issued Mar. 6, 1979; 4,181,753 issued Jan. 1, 1980; and 4,263,339 issued Apr. 21, 1981. The aging process is described in '297 as being aggravated by sulfur vacancies in the crystal lattice structure, which vacancies exhibit a negative charge and the presence of which Fischer believes promotes the diffusion of the positively charged copper ions within the grains of the phosphor. Fischer further believes that the copper out-diffuses to the surface and the electroluminescent mechanism becomes inoperative due to this form of aging. It is also clear that the aging is accelerated by the presence of moisture and electrolysis of the zinc and copper. As an intermediate step, Fischer treats his phosphor prior to coating by immersion in molten sulfur under heat and pressure in an autoclave. In '753, the disclosure is enhanced by the suggestion that metals may be added to the sulfur bath.
After the sulfur process, Fischer boils the treated powder in a concentrated phosphoric acid to form an insoluble zinc phosphate skin around each particle. The light transmissive qualities of this coating are not disclosed. Patent '753 discloses a further intermediate step, prior to the phosphoric acid bath, of heating the sulfur treated particles in hydrogen peroxide to convert the zinc sulfide surface to a zinc oxide surface and thereafter treating in phosphoric acid to convert the zinc oxide to the zinc phosphate coating.
Fischer also suggests that the particles can be glass coated, and states that the coating "can also consist of chemically vapor-deposited glass . . . produced by pyrolytic decomposition of metal-organic vapors." No example is given in Fischer of the metal-organic vapors, of any process for accomplishing the process, or of any lamp using such phosphors.
Attempts have been made to coat phosphor particles with glass, i.e., silicon dioxide, and include the U.S. Pat. No. 3,408,223 of Shortes, issued Oct. 29, 1968. Shortes was not concerned with the coating of phosphor particles for use in electroluminescent lamps and therefore was not concerned about extending the life of such a lamp or the phosphors therein, or the exclusion of water vapors from interaction with the phosphor particles. Rather, Shortes was concerned with the manufacture of a cathode ray tube phosphor which had selectively higher electron energy ionization thresholds, and disclosed the coating of phosphor particles by subjecting the phosphors to a tetraethoxysilane atmosphere under high temperature conditions, and subjecting the phosphor particles repeatedly to such atmosphere by recirculating the atmosphere and/or the phosphor particles therethrough so as to provide a silicon dioxide coating. Shortes contains no disclosure of the thickness or character of the coating, or of the efficacy of the use of such a treated phosphor particle in an electroluminescent environment.
United States Defense Department Technical Report AFFDL-TR-68-130 "Improving the Performance of Electroluminescent Lamps at Elevated Temperatures," July 1968 by Thompson et al, published by United States Air Force Flight Dynamics Laboratory, ASFC, Wright-Patterson Air Force Base, Ohio, discloses the coating of electroluminescent particles with various refractory materials including silicon dioxide, titanium dioxide, and beryllium oxide, among others. All of the coatings were applied by the pyrolysis of chemical vapors at atmospheric pressure in a heated fluidized bed reactor. The silicon dioxide coatings were applied by the decomposition of tetraethyl orthosilicate Si(OC.sub.2 H.sub.5).sub.4 or silicon tetrachloride SiCl.sub.4 with reactor temperature of 400.degree. C. There is no mention in this respect of silicon coated phosphors used in an electroluminescent lamp. Rather, the authors concentrated primarily on the use of titanium and beryllium coated phosphors in making, and then testing electroluminescent lamps at very high operating temperatures. The titanium coated particles tended to fuse together or cluster into groups of coated particles, and it was difficult to maintain the desired phosphor population in a lamp, apparently due to the shape of the particles and the quantity of coating included. Accordingly, the overall lamp brightness was reduced due to the reduced phosphor particle populations as compared to a conventional lamp using uncoated phosphors. The authors, however, indicated that the silicon dioxide coated zinc sulfide phosphor was given an accelerated water vapor resistance test, not otherwise described, and indicated that the material "looked like it showed promise."